Electric contacts



Jan. 23, 1962 w. RUPPEL 3,018,426

ELECTRIC CONTACTS Filed Oct. 19, 1959 id a Zia 21C gig 6! 236' i Zid"INVENTOR. Wozmmva KUPPEL 3,018,426 ELECTRIC CONTACTS Wolfgang Ruppel,Zurich, Switzerland, assignor to Radio Corporation of America, acorporation of Delaware Filed Oct. 19, 1959, Ser. No. 847,166

16 Claims. (Cl. 317237) This invention relates to novel ohmic electriccontacts to insulators and to novel space charge limited currentdevices.

Under normal conditions, electric current does not flow readily into andthrough an insulator. This is because: (I) potential barriers exist atthe electric contacts of the insulator and prevent large scale injectionof electrons or holes or both into the insulator; and (2) imperfections,which exist in the crystal structure of insulators, trap any initialelectron or hole flow and set up either or both a negative or a positivespace charge field which opposes further current flow.

If ohmic electric contacts (contacts which are free of contact potentialbarriers) are provided and if, in addition, the insulators aresufficiently free of trapping imperfections, the current flow of bothelectrons and holes in the insulator is limited in magnitude only by thespace charge field due to mobile carriers which flow into the insulator.Solid state devices of this type, are referred to herein as solid statespace charge limited current de vices, because they are analogous inmany respects to thermionic vacuum space charge limited current devices.In the vacuum devices, space charge limited currents are carried only byelectrons travelling through a vacuum. The solid state space chargelimited current devices are more versatile than the vacuum devicebecause in the former (1) either or both of two types of free chargecarriers, electrons and holes, may carry the current, (2) heat is notnecessary to produce the injection of charge carriers, (3) a vacuum isnot necessary, and (4) the electrical characteristics of the devices maybe modified by modifying the physical characteristics of the insulator.

The problem of providing suitable insulators for solid state spacecharge limited current devices is largely a problem of purification andcrystal growing of the material. Suitable crystals of cadmium sulfideand similar materials are available which have less than one trappingimperfection for every lattice sites. Such crystals are insulating andhave an apparent volume resistivity greater than 10 ohm cm.

Also, the prior art provides electric contacts to crystals of the typedescribed above which are ohmic to electron current flow but arerectifying to hole current flow. Such electric contacts are made bycontacting, as by pressing, indium or gallium metal against theinsulator. There are no heating or forming steps involved. Upon removalof the contact from the insulator, the insulator surface appears to befree of chemical and physical action from the contact. However, theprior art does not provide similar electric contacts which are ohmic tohole current flow.

' An object of this invention is to provide ohmic electric contacts forthe injection of positive charge carriers or holes into insulators.

A further object is to provide to insulators, electric contacts whichare ohmic to the flow of the hole currents and are rectifying to theflow of electron currents.

Another object is to provide electric contacts which are ohmic to theflow of holes for insulating single crystals of cadmium sulfide andsimilar materials.

In general, the electric contacts of the invention herein include atellurium electrode in physical contact with the surface of acrystalline insulator body selected from the group consisting of cadmiumsulfide, cadmium selenide, zinc sulfide, zinc selenide and zinc oxide.It has been lice 2 found unexpectedly that such a combination providesan ohmic contact for hole current flow between the electrode and theinsulator, while being highly resistive to electron current flowtherebetween. In a preferred embodiment, a fiat surface of a telluriumbody is placed against the face of a single crystal of insulatingcadmium sulfide to provide the ohmic contact.

The invention herein includes devices comprising the foregoingcombination and, in addition, a second electrode, such as indium,gallium, tin, lead, or combinations thereof, which produces a contactwhich is ohmic to electron current flow into said body. Such devices,with the two electrodes biased in the forward direction so that bothholes and electrons are injected into the insulator, provide the maximumin electric current flow by providing a cross neutralization of thespace charge produced in the insulator body near both contacts due tothe injected carriers. Where the insulator is properly selected for thepurpose, the injected carriers recombine radiatively in the insulatorbody to produce band gap electroluminescence.

In addition, the foregoing devices may include also control electrodescontacting the insulator body. Such electrodes, when biased in thebackward direction and with a suitable signal voltage applied thereto,may modulate either or both the electron and hole current flow bycontrolling the magnitude of the space charge opposing the flow currentthrough the insulator. A suitable control electrode for electroncurrents comprises a material suitable as the hole injector contact. Asuitable control electrode for hole current comprises a materialsuitable as the electron injector contact.

The novel features of the invention are set forth in greater detail inthe following description and the accom panying drawing in which:

FIGURE 1 is a partially elevational, partially sche-s matic view of anembodiment of the invention including a device and circuit therefore,

FIGURE 2 is a graph illustrating a typical voltage characteristic of thedevice of FIGURE 1.

FIGURE 3 is a partially elevational, partially schematic view of anotherembodiment of the invention including a device and circuit therefor.

FIGURE 4(a) and FIGURE 4(b) are respectively elevational and plan viewsof a device including a control electrode for electron current.

FIGURE 5(a) and 5(1)) are respectively elevational and plan views of adevice including a control electrode for hole current, and

FIGURE 6(a) and 6(b) are respectively elevational and plan views of adevice including control electrodes.

I tacts of the invention is illustrated in FIGURE 1.

for both electron and hole currents.

. Similar reference numerals are applied to similar structuresthroughout the drawing.

Example.A simple device including the ohmic con-v The device comprises asingle crystal 21 of insulating cadmium sulfide about 0.01 mm. thick andhaving a volume resistivity of about 10 ohm cm. The crystal 21 has twoplane opposed surfaces and is prepared by any convenient process, forexample the vapor phase technique described by R. H. Bube and S. M.Thomsen in the Journal of Chemical Physics, volume 23, page 15 (1955)..A wafer of tellurium having a plane surface is attached to a firstbrass support 29 with silver paste 27 or other electrically conductingadhesive. An indium body 25 of about 0.1

mm. radius is pressed against a face of a second brass support 31. Theplane surface of the tellurium wafer 23 is pressed against one face ofthe crystal 21 as with a spring 33 and, at the same time, the indiumbody 25 is pressed against the opposed face of the crystal 21 as with aspring 34 with a force of the order of grams.

A battery 37, a reversing switch 36, and a variable resistor 35 areconnected in series to the brass supports 29 and 31, so that thetellurium wafer 23 and the indium layer 25 are biased in oppositepolarity. The forward direction for biasing the contacts with respect tothe crystal 21 for injecting holes from the tellurium contact 23 and forthe injecting electrons from the indium contact 25 is with the telluriumcontact 23 biased positively and the indium contact 25 biasednegatively.

The curve 39 of FIGURE 2 illustrates a typical voltagecurrentcharacteristic of the device of FIGURE 1. The switch 36 is adjusted toapply the voltage polarity to the crystal 21 in the backward directionand the resistor 35 is varied to provide a range of voltages. The totalcurrent is very low over the entire range of voltages below breakdown inthe backward direction. When the contacts 23 and 25 are biased in thebackward direction, both contacts are non-injecting. No carriers diffusefrom the contacts into the insulator, and the current is that of aninsulator to which blocking contacts are applied.

The switch 36 is readjusted to apply a voltage polarity to the crystal21 in the forward direction, and the resistor 35 is varied to provide arange of voltages. The current remains low over a range of low voltagesin the forward direction. This low current is an ohmic current whichresults from free carriers originating in the body due to thermal actionin the body. Generally the ohmic current is a linear function of theapplied voltage. When the bias in the forward direction is sufiicient tocause substantial injection of holes or electrons or both into theinsulating cadmium sulfide crystal 21, the total current rises steeplyand exceeds the ohmic current by several orders of magnitude. Therapidly increasingportion of the curve 39 is due to injected carriersand is referred to as the space charge limited current region. Injectionof carriers starts as soon as a forward bias is applied to the contacts,and becomes substantial at about 200 volts. At about 50 volts, the spacecharge limited current is of the order of one milliampere, or 0.4ampere/cmfi. The space charge limited current is limited only by thespace charge of injected carriers in the crystal 21.

In the forward direction, the space charge limited current flow may beaccompanied by emission of band gap light (electroluminescence). In thecase of the cadmium sulfide crystal of the example the light is green,peaking at about 5200 A. at room temperature. It is believed thatinjected electrons and holes recombine across the band gap of theinsulator yielding one photon for each recombination. Thedevices may beoptimized so that they produce more efiicient electroluminescence.

' Another structural arrangement is illustrated in FIG- URE 3. A layer23a of tellurium is evaporated upon one face of a crystal 21a ofinsulating cadmium sulfide about 10 microns thick. The tellurium contacttherein has an area of aboutLO square mm. The tellurium layer 23a isproduced by placing a quantity of tellurium in a container, supportingthe crystal above and spaced from the container about 20 cm., evacuatingthe region to about 10 ofHg and then heating the tellurium above itsmelting point to evaporate it, for example, at about 500 C. Then a layer25a of indium is evaporated on the opposite face ofthe crystal 21a.Electrical connections are made by spring clips 33a and 34a bearingagainst the evaporated layers 23a and 25a respectively. The device isoperated as described with respect to the device of FIGURE 1. Thecurrent drawn in the steady state at 5 volts is about 4.0 amperes (0.4ampere/cm?) in the forward direction and about 4.0x 1'0- amperes (4.01() arnperes/cm?) in the backward direction. The device is an'efficientrectifier with relatively low leakage in the back direction.

Ohmic contacts for the flow of hole current into insulators may beobtained by using the crystalline variety of tellurium. The telluriumshould be as pure as possible. Certain materials, however such asselenium and sulfur,

when they are present in relatively small amounts, have no adverseeffect on the tellurium as an ohmic contact for hole current.

The tellurium electrode of the embodiment of FIG-- URE 1 may be producedin any desired shape by any of the commonly known methods, for examplerolling, punching, and stamping. The electrode in its simplest form is asingle composition that has been shaped as desired. Alternatively, theelectrode material may be coated or plated on some other material thatwill serve as a support. For example, suitably shaped sheet nickelhaving a layer of tellurium on one side makes a good ohmic contact.

After the tellurium electrode is shaped, the surface of the electrode isapplied to the surface of the insulator. All that is necessary is thatthe two surfaces are in indmate physical contact with one another. Ifthe electrode material is soft enough with respect to the insulatorbody, merely placing the two surfaces against one another with theslightest pressure will produce a good ohmic contact as well as goodphysical contact. In other cases, pressure and heating are used tofacilitate intimate physical contact between the surfaces. If heating isemployed, a nonreactive atmosphere is preferred. After the contact ismade, the heat and pressure are removed. While heating may be used tofacilitate producing good contact, it should be clear that it is usedfor the purpose of making intimate physical contact between theelectrode and the body surface, and that it is not for the purpose ofdiffusing the electrode material into the insulator body. When theelectrode is removed from the insulator body after a successful ohmiccontact has been made, there is no sign of the previous contact on thesurface of the insulator, nor does a subsequent contact prefer theprevious contact area.

Successful ohmic contacts for hole current injection into insulators mayalso be obtained by producing the tellurium electrode directly on theinsulator body by any of several well known methods. For example, thetellurium electrode may be formed by evaporating, sputtering, orspraying tellurium on the surface of the insulator body.

Good ohmic contacts for space charge limited hole currents may be madewith the tellurium electrodes de scribed above to bodies of materialwhich are substantially free of charge carriers in their volume,asopposed to those bodies which have substantial amounts of free chargecarriers, either electrons or holes, already present therein. Materialswhich are substantially free of free charge carriers are frequentlyreferred to as insulators. However, the range of resistivity which isincluded is not sharply defined. For example, cadmium sulfide isgenerally classed as an insulator when its resistivity is greater than10 ohm-cm, and as a semiconductor when its resistivity between 10 and 10ohm-cm. Ohmic contacts for hole currents may be made with tellurium inthis entire resistivity range; that is, in the range of resistivitygreater than 10 ohm-cm. However, it is preferred to use insulators withhigher resistivities, preferably greater than 10 ohm-cm. Further, it ispreferred that the insulator body have a minimum of impurities andtraping imperfections. 'It is noteworthy that the ohmic contactsdescribed herein are intended for use in bodies which are not sources offree charge carriers, but which merely provide a medium whosecharacteristics are similar in some respects to a vacuum into whichinjected free charge carriers interact with one another. Some typicalinsulators are cadmium sulfide, cadmium selenide, zinc sulfide, zincselenide, and zinc oxide.

The ohmic contacts to insulators, for the injection of electroncurrents, may be prepared in a manner similar to that of preparing thetellurium contacts, except that another material is substituted for thetellurium. The ohmic contacts for the injection of electron currents mayconsist essentially of a metal or combination of metals selected fromthe group consisting of indium, gallium, tin, and lead. Other types ofcontacts that are ohmic for electron current flow to insulators may beused.

Another embodiment of the devices of the invention may also includeelectrodes attached to the insulator body for controlling one or both ofthe space charge limited electron or hole current in the device. FIG-URE 4(a) is an elevational view and FIGURE 4(b) is a plan view of adevice including an electrode for controlling the space charge limitedelectron current in a device of the type described with respect to FIG-URE l.

The device of FIGURES 4(a) and 4(b) includes a single crystal 21b ofinsulating cadmium sulfide contacted at one end by an indium electroninjector contact 25b which is ohmic to electron current flow, and atellurium hole injector contact 2312 which is ohmic to hole currentflow. As shown in FIGURE 4(b), batteries 43 and 45 and a load resistor47 are connected in series between the injector contacts 23b and 25b,biasing the contacts in the forward direction for simultaneous injectionof both electrons and holes. An electron current control electrode 41 oftellurium contacts the crystal 21b at a position spaced from, butadjacent the electron injector contact 25b. In operation, the electroncurrent control electrode 41 is biased in the backward direction withrespect to the crystal 21b as with a battery 55 and is not injecting. Itis noteworthy that the electrode 41 is back biased with respect to thecrystal 21b with low positive voltages as well as with negativevoltages. A signal voltage from a source 51 is coupled to the electroncurrent control electrode 41 through a coupling transformer. The signalvoltage produces in the crystal 21b a variable electric field in thenegative space charge region adjacent to the electron injector contact25b. This is the region of the crystal 21b into which the electrons ornegative charge carriers are injected. The electric field from theelectron current control electrode 41 builds up or reduces the negativespace charge present in the region, which in turn varies correspondinglythe electron current output which may be monitored at terminals 49 as avoltage across the load resistor 47. The output of the device shows botha current gain and a power gain over the input to the device. The inputsignal on the control electrode appears in the output of the device.

FIGURE 5(a) is an elevational view, and FIGURE 5 (b) is a plan view, ofa device including an electrode for controlling the space charge limitedhole current in the device. The device of FIGURE 5 (b) is similar instructure and operation to the device of FIGURE 4(b) and includes asingle crystal 210 of insulating cadmium sulfide contacted at one end byan indium electron injector contact 250 which isohmic to electroncurrent, and a tellurium hole injector contact 230 which is ohmic tohole current. As shown in FIGURE 5 (b), batteries 43a and 45a and a loadresistor 47a are connected in series to the ohmic contacts 230 and 250,biasing the contacts in the forward direction for simultaneous injectionof both electrons and holes. A hole current control electrode 61 ofindium or gallium contacts the crystal 210 at a position spaced from,but adjacent, the hole injector contact 23c.

In operation, the hole current control electrode 61 is biased in thebackward direction with respect to the crystal 21c as with a battery 75and is not injecting carriers. A signal from a source 71 is coupled tothe positive control electrode 61 through a coupling transformer 73. Thesignal voltage produces in the crystal 210 a variable electric field inthe positive space charge region; that is, the region of the crystal 21cinto which holes or positive charge carriers are injected. The electricfield from the hole current control electrode 61 builds up or reducesthe positive space charge present in the region, which variescorrespondingly the hole current. The output signal, which follows theinput signal, may be read at terminals 49a as a voltage across 6 theload resistor 47a, in a manner similar to that described with respect tothe operation of the device of FIGURE 4(b).

The device of FIGURES 6(a) and 6(b) includes both control features ofthe devices of both FIGURES 4(b) and 5 (b). The device of FIGURE 6(b) issimilar in structure and operation to the devices of FIGURES 4(b) and5(b) and comprises the single crystal 21d of insulating cadmium sulfide,a hole injector contact 23d, electron injector contact 25d, a negativecontrol electrode 41b and a positive control electrode 61b. The deviceis connected and operated as in FIGURES 4(b) and 5 (b). The circuitincludes means for biasing the injector contacts 23d and 25d forsimultaneous electron and hole injection. The electron current controlelectrode 41b and the hole current control electrode 61b are connectedto means for applying a signal to one or both of the electrodes 41b and61b including batteries 55b and 75b, a coupling transformer withvariable center tap 87 on the secondary and an input signal source 81What is claimed is:

1. An electrical device comprising a crystalline body of insulatingmaterial selected from the class consisting of cadmium sulfide, cadmiumselenide, zinc sulfide, zinc selenide, and zinc oxide, and at least oneelectrode consisting essentially of tellurium in contact with said body,said electrode providing ohmic contact for hole current fiow betweensaid one electrode and said body.

2. An electrical device comprising a crystalline body selected from theclass consisting of cadmium sulfide, cadmium selenide, zinc sulfide,zinc selenide, and Zinc oxide and at least one electrode consistingessentially of crystalline tellurium in contact with said body, saidelectrode providing ohmic contact for hole current flow between said oneelectrode and said body.

3. The device of claim 2 wherein said body is cadmium sulfide.

4. The device of claim 2 wherein said body is cadmium selenide.

5. The device of claim 2 wherein said body is zinc sulfide.

6. The device of claim 2 wherein said body is zinc selenide.

7. The device of claim 2 wherein said body is zinc oxide.

8. An electrical device comprising a single crystal body of materialselected from the class consisting of cadmium sulfide, cadmium selenide,zinc sulfide, zinc selenide, and zinc oxide and at least one electrodeconsisting essentially of crystalline tellurium in contact with saidbody, said electrode providing an ohmic contact for hole current flowbetween said one electrode and said body.

9. An electrical device comprising a single crystal of cadmium sulfidehaving a resistivity greater than 10 ohm-cm. and being relatively freeof trapping imperfections, and an electrode consisting essentially ofcrystalline tellurium contacting said crystal, said electrode providingohmic contact for hole current flow between said electrode and saidcrystal.

10. An electrical device comprising a crystalline, insulating bodyselected from the group consisting of cadmium sulfide, cadmium selenide,zinc sulfide, zinc selenide, and zinc oxide, a first electrodeconsisting essentially of crystalline tellurium contacting said body,said first electrode providing ohmic contact for hole current flowbetween said first electrode and said body, and a second electrodecontacting said body and spaced from said first electrode, said secondelectrode providing ohmic contact for electron current flow between saidsecond electrode and said body.

11. The device of claim 10 wherein said second electrode consistsessentially of a metal or combination of metals selected from the groupconsisting of indium, gallium, tin, and lead.

12. The device of-claim '10 including a third electrode contacting saidbody intermediate said first and second electrodes for controlling oneof said currents.

13. The device of claim 12 wherein said third electrode is forcontrolling said hole current and consists essentially of a metal orcombination of metals selected from the group consisting of indium,gallium, tin, and lead.

14. The device of claim 10 including a third and a fourth electrodecontacting said body intermediate said first and second electrodes, saidthird electrode for separately controlling said hole current and saidfourth electrode for separately controlling said electron current.

15. An electrical device comprising a single crystal of insulatingcadmium sulfide having a resistivity greater than 10 ohm-cm. and beingrelatively free of trapping imperfections, a first electrode consistingessentially of crystalline tellurium contacting said crystal, said firstelectrode providing ohmic contact for hole current flow between saidfirst electrode and said crystal, and a second electrode consistingessentially of a member of the group consisting of indium, gallium, tin,lead and combinations thereof, contacting said crystal, said secondelectrode providing ohmic contact for electron current flow between saidsecond electrode and said crystal.

16. A device comprising a single crystal of insulating cadmium sulfideand being relatively free of trapping imperfections, a first electrodeof crystalline tellurium in contact with said body, and a secondelectrode of indium in contact with said body.

References Cited in the file of this patent UNITED STATES PATENTS

15. AN ELECTRICAL DEVICE COMPRISING A SINGLE CRYSTAL OF INSULATINGCADMIUM SULFIDE HAVING A RESISTIVITY GREATER THAN 1010 OHM-CM, AND BEINGRELATIVELY FREE OF TRAPPING IMPERFECTIONS, A FIRST ELECTRODE CONSISTINGESSENTIALLY OF CRYSTALLINE TELLURIUM CONTACTING SAID CRYSTAL, SAID FIRST